Antenna



Feb. 3, 1959 Filed Sept. 13, 1956' F. G. R. WARREN ETAL 7 Sheets-Sheet 1 FFED HFC' F55@ if@ IN V EN TOR5 Feb- 3, 1959 F. G. R. WARREN ET AL 2,872,679

ANTENNA 7 Sheets-Sheet 2 Filed Sept. l5, 1956 3S o o Feb- 3Q 1959 F. G. R. WARREN ETAL 2,872,679

ANTENNA 7 Sheets-Sheet 5 Filed Sept. 15, 1956 i W a M WMU H- H a 7/ a m W 3f W fj ...Ami nl W m INVENTORS 5PM/a5 f /Pam Wav/PMN Feb- 3 1959 F. G. R. WARREN ETAL 2,872,679

ANTENNA Filed Sept. 15, 1956 7 Sheets-Sheet 4 7o Raf/v51?- N/ #mmf/77M Feb. 3, 1959 F. G. R. WARREN ET AL 2,872,679

ANTENNA 7 Sheets-Sheet 5 Filed Sept. 15, 1956 INVENTORS mwa/cfs 6, 5055 W/l/mtw 2oz/509W Maz/9K m... Ml IUIHII Feb. 3, 19,59 F. G. R. WARREN ETAL 2,872,679

ANTENNA 'I Sheets-Sheet 6 Filed Sept. 15, 1956 l TTORNEY Feb. 3, 1959 F. G. R. WARREN ET AL 2,872,679

ANTENNA Filed Sept. 15, 1956 7 Sheets-Sheet 7 ANTENNA Francis G. R. Warren and Zdzislaw A. Melzak, Montreal, Quebec, Canada, assignors to Radio Corporation of America, a corporation of Delaware Application September 13, 1956, Serial No. 611,407

s claims. (ci. 343-754) The present invention relates to an antenna and in particular to a scannable antenna of improved design which is particularly useful for microwave transmission and/or reception.

In many types of sector scanning systems, a source, such as a horn feed element, moves along a locus to produce the beam scanning motion. The locus is usually approximated by a circular arc and the source is normally directed at the center of the radius of curvature of the arc as it moves along the arc. However, this center may not, and usually does not, coincide with the center of the final aperture through which waves are emitted or received. For example, in the parallel plate systems of the Schwarzschild, Robinson, R.2R scanners or similar types, the effective system focal length (the distance through the parallel plates from the feed arc to the center of the final aperture) is normally roughly equal to the final aperture length in order to obtain a reasonable degree of scanning Without deterioration ofthe beam. (The Robinson scanner is described in the Radiation Laboratory Series, volume 26, section 2.15; the R.2R scanner is described in RCA Review, volume IX, No. 4, December, 1948; a Schwarzschild scanner of improved design is described herein.)

In the design of a narrow beam antenna the final aperture must necessarily be large. This, in turn, increases the system focal length, as explained above. If the feed arc radius were made as long as the increased system focal length, the diameter of the full circle continuation of the feed arc might be of the order of twice the aperture size, and the resulting apparatus would be extremely bulky. (Note that the feed element motion is normally along the full circle continuation of the feed arc.) Additionally, if the feed arc radius were made as long as the focal length, the feed arc length would be only a small portion of the entire feed circle, approximately equal to the scan angle. In a specific application to be described below in which it is desired to scan through a 12 sector, this would amount to a 12 arc, and for continuous scanning, 24 switched feeds would be necessary, assuming a dead time of 3 for each switching operation. The use of such a large number of feeds is, of course, uneconomical and mechanically complex.

ln examples of the previous art, a feed circle radius is chosen that is longer than the optimum feed circle radius but shorter than the focal length, as a compromise between proper illumination of the aperture and correct focus position. The result is a considerable deterioration of the beam at increasing scan angles. That is, the beam characteristics (width, side lobes, etc.) are increasingly adversely affected as the scan angle increasingly diverges from zero degrees. Itis a general object of this invention to provide, for an antenna, an improved feed system that will enable a source of microwave energy to be moved along an arc closely corresponding to the true locus of focus of a collimating system and at the same time to the center of the final aperture of the system.

2,872,679 Patented Feb. 3, 1959 Another object of this invention is to provide an improved sector scannable, narrow-beam antenna of the parallel-plate type in which there is substantially no degeneration or deterioration of the beam characteristics during scanning.

Another object of this invention is to provide, for a parallel-plate, scannable antenna system, an improved means for simultaneously accomplishing the following;

(a) Moving the feed horn along a circle;

(b) Changing the horn orientation with respect to the tangent to that circle; and

(c) Maintaining good electrical contact, with respect to radio-frequency transmission and reception, between the `horn aperture and the parallel plates.

The antenna of the present invention includes an aperture through which energy may be received or emitted and a feed element which is movable along the antenna feed arc for scanning the received or emitted beam. The center of the radius of curvature of the feed arc is displayed from the center of the aperture. The antenna is provided with means for directing the beam emitted or received by the feed element toward the center of the antenna aperture at all positions of the feed element along the feed arc.

In a specific form of the invention, the antenna comprises a folded, parallel-plate antenna of the Schwarzschild type having an arc-shaped aperture along which a feed element may move and a final aperture through which energy is emitted and received. The feed element comprises a parallel-plate portion, hereinafter termed a stator, having an aperture which corresponds in shape to a portion of the arc-shaped aperture. This stator aperture communicates with the parallel plates of the Schwarzschild antenna at the arc-shaped aperture, as the stator moves along the latter. A rotor is mounted in the stator. The rotor is formed with an internal waveguide which communciates at one end with the stator aperture, and communicates at the other end with a waveguide which leads to the receiver and/ or transmitter of a radar system. As the feed element moves along the feed arc, the rotor moves within the stator in a manner such that the feed element beam is always directed at the center of the nal aperture of the Schwarzschild antenna.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing in which:

Figures 1a and 1b are sketches of an optical system which is useful in explaining how the antenna of the present invention operates;

Figure 1c is a sketch of a parallel-plate antenna system according to this invention;

Figure 2 is a partially cut-away, plan view of an embodiment of an antenna in accordance with the present invention;

Figures 3a and 3b together comprise a cross-sectional view along line 3x1-3b of the antenna shown in Figure 2;

Figure 4a is a plan view of the feed assembly of the antenna shown in Figures 2, 3a and 3b;

Figure 4b is a cross-section taken along line ib-4b of Figure 4a;

Figure 4c is a cross-section taken along line 4c-4c of Figure 4b;

Figure 5 is a perspective View of the Schwarzschild parallel-plate structure and the feed system, both according to the invention; and

. Figure 6 is a sketch of the beam patterns obtained at different angles of scan with the antenna of the present invention. This figure also shows the beam patterns obtained when the feed system is maintained directed at the center of the radius of curvature of the feed arc, as in prior art systems.

"assauts Similar reference characters `are applied to similar elements through the drawings.

Throughout the discussion which follows, the antenna of the invention will be described in terms of a transmitting antenna. This is to simplify the discussion of the invention. It should be appreciated that the system is also operative as a receiving antenna and that in a practical radar system, the antenna may be used both for transmitting and receiving.

Referring to Figure la, lens designates the final aperture of a parallel-plate antenna system. A source of radio-frequency waves is moved along feed arc 12 to produce the beam scanning motion. The center 14 of the radius of curvature of the feed arc is displaced from the center 16 of lens 1i). ln a practical antenna system dcsigned for operation in the 35,000 megacycles region. the focal length of the antenna (analogous to the distance between the center of the feed arc and the center of the lens) is about 78 inches, whereas the radius of the feed arc iS less than ll inches.

lf, as in the prior art systems, the source of electromagnetic waves is continuously directed at the center of the radius of curvature of the feed arc, the type of operation shown in Figure lb will result. When the source (shown here as an arrow 18) is at the center of the feed arc, the axis of the beam it emits intersects the center 16 of the lens. The beam axis is indicated in Figure lb by dashed line 2i) and the outer extremities of the beam by dashed lines 22. As the source 18 moved along the feed arc from its center position, the beam axis is progressively displaced from the center 16 of lens 10. As can be seen from Figure 1b, when the source reaches position 18', beam axis Ztl is now displaced a substantial amount from lens center 16 and a portion x of the beam pattern actually does not go through the lens at all. The result of this type of scanning motion is serious deterioration of the beam passing through the lens. v

Some prior art systems have attempted to overcome the problem dealt with in the foregoing paragraphs by so designing the beam focusing system that the center of the feed arc coincides with the center of the final aperture. One system of this type is described in Patent No. 2,585,562, issued on February l2, 1952, to W. D. Lewis. However, as indicated by the antenna patterns shown in Figure 16 of the disclosure, this solution is not entirely satisfactory. Moreover, this solution requires parallelplate structures of special and costly design.

The antenna of the present invention includes a parallel-plate structure that incorporates a long final aperture to produce a narrow beam width. Although the focal length of the system is slightly longer than this aperture, the radius of curvature of the feed are has kept to a small fraction of this length and thereby a relatively small compact unit has been achieved. However, the feed element is so oriented during scanning that the axis of the beam emitted by the feed element is always directed to the center of the aperture.

Figure lc illustrates how the parallel-plate system of the present invention operates. The folded-parallelplate structure is shown schematically as including an arc-shaped aperture Ztlil, along which the feed element 202 may move, and a final aperture 204, through which energy may be received or emitted. The feed element includes a stator 266 having an aperture 208 shaped like that of the parallel-plate aperture 200 and a rotor 209. The rotor is formed with an internal horn, shown by lines 210, which communicates with aperture 293. As the throat 214 (the center of phase of the feed pattern) of the horn moves along the feed arc, the stator continuously maintains its electrical contact with the aperture 2G43, as indicated by the full line View of the horn and the dotted line view 216 of the horn in another position. The rotor 209 rotates within the stator as the latter moves around the aperture Ztlil in a manner such that the axis 218 of the feed pattern always intersects the center 220 of the final aperture 264. The limits of the feed pattern, when feed element 202 is in a position corresponding to the center of the angle scanned, is shown by solid lines 222. With the horn in the position shown by dashed lines 216, the feed pattern is as indicated by dot-dash lines 22d. If the rotor were not rotated as the feed element moved along the feed arc, most of the beam would miss the final aperture, as shown by dashed lines 226.

lt may be noted that Figure lc is drawn as though there were no focusing between the source and the final aperture. All focusing action takes place at the final aperture. Where focusing occurs at intermediate points, as in the Schwarzschild antenna, the ray tracing is more complicated, but the principle is the same.

Figures 2, 3a and 3b show a practical system in accordance with the present invention. Still considering the system as a transmitting system, energy from a transmitter (not shown) enters waveguide Sil and is applied to the fixed sector 32 of a switch ring assembly 34. This switch ring assembly is described in greater detail in copending application Serial No. 529,232, led August 18, 1955 by L. l. Bickley and A. R. V. Roberts, now abandoned. The switch ring, in brief, comprises a fixed sector 32, and a rotatable outer ring 36. The outer surface of the sector 32 is formed with a circumferential groove 4@ and the inner surface of ring .36 is formed with a corresponding, inner circumferential groove 42. he two grooves together form a waveguide of variable length. Chokes at the ends of the waveguide prevent energy from leaking out of the waveguide. Thus, energy applied to the waveguide via input waveguide 30 travels through the internal, variable length waveguide between the inner and outer portions of the switch ring in the direction indicated by arrow 44 (Figure 2).

The outer ring of the switch ring assembly is continuously rotated. The rotating means include a motor 46, pulleys 48 and belts 50 and 52. The nal belt 52 drives an annular structure 54 which is permanently connected by means of bolts 60 to the outer ring 36 of the switch ring assembly.

A plurality of feed assemblies 62 are permanently connected by means of output waveguides 64 to the outer ring of the switch ring assembly. The rotors are shown in greater detail in Figures Litz-4c to which the reader is now referred.

Each feed assembly includes a stator portion 70 and a rotor portion 72 which is rotatable with respect to stator 74). Energy from waveguide 64 is fed to a waveguide 73 in the stator portion of the yfeed assembly. The energy passes through channels 74 and 76 to the upper portlon 77 of the rotor assembly. In a preferred form of the invention bends 78 are spaced 3A of a wave length from one another as are bends 80 to minimize reflections at the bends. Choke slots 80a and Stlb prevent the leakage of energy from waveguide channels 74 and '76, respectively. In each case, the distance from the shortclrcuted end of the slot to the mitered bend (7S or dll) 1s one-half wave length at the operating frequency, whereby the opening between stator and rotor at the mitered bend appears as a short circuit. Energy in waveguide portion 77 of the rotor passes through the circular bend 81 in the rotor and out through horn 82. The horn may be seen somewhat more clearly in the phantom view of Flgure 4a. As the energy is emitted from the horn, 1t passes through parallel plate structure 3d of the stator. The parallel plate structure is sufficiently large along dimension 85 (Figure 4a) that regardless of the position of the rotor relative to the stator, the energy emitted from the horn passes through parallel plates 84 without being obstructed.

Referring now to Figures 3b and 5, it can be seen that the parallel plate portion 84 of the feed assembly meshes with the feed arc 86 of the Schwarzschild antenna 90. As the outer ring of the switch ring assembly is rotated, one of the feed assemblies 62 travels along the feed arc 86. The one of the feed assemblies which feeds the Schwarzschild antenna depends, of course, on the position of the feed assembly relative to the stationary Sector 32 of the switch ring. A plurality of feed assemblies are employed to obtain a relatively continuous radiation (or reception) of energy using a uniform rotational speed. It will be appreciated that the invention is equally operative with only a single scanning assembly or with 2, 3, 4 or more such feed assemblies. However, if only one feed assembly were used with the antenna design illustrated here, then the antenna would scan for only about one-fourth of the time of rotation of the feed mechanism. To make full use of the one feed assembly a mechanism could be designed to oscillate this one feed backwards and forwards along the feed arc, but for a high speed scanner this would be an undesirable motion. On the other hand, more or less than the present four feeds can be used to reduce or increase the present angle of scan, respectively, always provided that the increase in scan does not produce intolerable deteriorations in the beam characteristics.

The means for rotating the rotor of the feed assembly so that its horn portion 82 is always directed at the center of the final aperture 92 of the Schwarzschild antenna includes a lever 94 permanently connected to the central axle 96 of the rotor, and a ball-bearing mounted roller 98 rotatably mounted at one end of the lever. The rotor rides on a cam surface 100 (Figure 2). The cam surface is so shaped that as the feed assembly moves, the horn end 82 of the rotor always points at the center of the final aperture 92 of the Schwarzschild antenna. Roller 98 is maintained in firm engagement with cam surface'100 by centrifugal force during the rotation of ring 36,

Immediately above roller 9S is an arc-shaped sector piece 102 (Figures 4a and 3b) which forms a skid and is fixed to the rotor axle 96. This skid rides adjacent to the annular cam-shaped surface 104. The purpose of skid 102 and annular cam-shaped sufrace 104 is merely to prevent the feed assembly from moving out of its prescribed course when at stand-still. Thus, as can be seen from Figures 3a and 3b, when the switch ring and feed assembly are rotating, roller 98 engages annular cam surface 100 and skid 102 is spaced from annular cam surface 104. When the feed assembly is at rest, surface 104 prevents any excessive amount of movement of the feed assembly.

Referring to Figures 3a and 3b, energy emitted from the horn end 82 of the rotor passes through parallel plate stator structure 84, and through the folded waveguides 110 of the Schwarzschild antenna. The final aperture of the Schwarzschild antenna is expanded in the form of a horn 92. Energy is emitted from the horn and illuminates parabolic cylinder 112. Plate 114 is formed of a dissipating material and its function is to reduce any side lobes due to spurious reflections.

Figure 6 illustrates the performance of the antenna of this invention. Beam pattern 120 is obtained with the horn portion 82 of the rotor centered at the center 122 (Figure 5) of the Schwarzschild antenna feed arc 86. It will be noted that the beam width is on the order of 0.3 (3 db down points). The solid .line 124 is the beam pattern obtained at a scan angle of 3 with the horn portion 82 of the rotor directing its beam at the center of the final aperture of the Schwarzschild antenna. There are no side lobes of any consequence, and the beam pattern remains very narrow-less than 1. The dotted line 126 is the pattern obtained at a scan angle of 3 with the horn of the rotor directed at the center of curvature of the feed arc of the Schwarzschild antenna. It is to be noted that even at this small angle of scan, serious deterioration of the beam pattern is beginning. Thus, the gain is lower `and side lobes are beginning to appear. Also, the beam width is increasing. This effect is very pronounced in the next beam pattern shown, that is, with the scan angle at 6. The pattern shown by solid line 128 is still of extremely high quality, with no deterioration of any consequence. This is the pattern obtained with the feed horn correctly oriented, that is, so that the feed pattern is directed at the center of the final antenna aperture. Dashed line 130 is a pattern obtained at Ia scan angle of 6 with the feed horn oriented to direct the feed beam at the center of the radius of curvature of the feed arc. Notice the decrease in beam gain and the very substantial increase in beam width and side lobes.

What is claimed is:

1. An antenna formed with an aperture through which energy may be received or emitted, and having a feed arc along which a directive beam may be scanned, the center of the radius of curvature of said feed arc being displaced from the center of said aperture, a feed element for receiving or emitting energy in a directive beam, said element being movable along said feed arc for scanning the received or emitted beam, and means operatively associated with said feed element for effectively directing said beam at the center of said aperture at all positions of said feed element along said feed arc.

2. An antenna formed with an aperture through which energy may be received or emitted, and having a feed arc along which a directive beam may be scanned, the radius of curvature of said feed arc being substantially shorter than the effective distance between said feed arc and the center of said aperture, a feed element for receiving or emitting energy in a directive beam, said element being movable along said feed arc for scanning a received or emitted beam, and means operatively associated with said feed element for effectively directing said beam at the center of said aperture at all positions of said feed element along said feed arc.

3. An antenna formed with an aperture through which nergy may be received or emitted, and having a feed arc along which a directive beam may be scanned, the radius of curvature of said feed arc being substantially shorter than the distance between said feed arc and the center of said aperture, a feed element for receiving or emitting energy in a directive beam, said element being movable along said feed arc for scanning a received or emitted beam, and mechanical means operatively associated with said feed element for effectively directing said beam at the center of said aperture at all positions of said feed element along said feed arc.

4. An antenna including a parallel plate structure terminated at one end in an aperture through which energy may be emitted or received and terminated at another end in an arc-shaped aperture along which a feed element may move, the radius of curvature of said arc being a fraction of the focal length of said antenna, that is, a fraction of the length between the parallel plates from the feed arc to the aperture; a feed element movable along said feed are and arranged to emit waves into or receive waves from the parallel plate structure at the feed arc aperture thereof; and means operatively associated with said feed element for effectively directing the axis of the beam received or emitted by the feed element at the center of said aperture in all positions of said feed element along said feed arc.

5. An antenna as set forth in claim 4, wherein said parallel plate structure comprises a folded, parallel plate structure.

6. An antenna as set forth in claim 5, wherein said parallel plate structure comprises the parallel plate structure of a Schwarzschild type antenna.

7. An antenna including a folded, parallel plate structure terminated at one end in an aperture through which energy may be emitted or received and terminated at another end in an arc-shaped aperture along which a feed element may move, the radius of curvature of said are geraete being substantially shorter than the distance through the parallel plates Vfrom said arc-shaped aperture to said aperture through which energy 4may be emitted or received; a feed element for radiating or receiving energy in a directive beam, said element being mounted to be movable along said feed arc in such manner that its beam passes through said arc-shaped aperture as the feed element moves along said arc, said feed element including a parallel-plate stator portion, the parallel plates of which align with the corresponding parallel plates of the arc-shaped aperture as the feed element moves along said arc-shaped aperture, and a rotor portion mounted in said stator portion formed with an internal waveguide therein which communicates at one end with the parallel plates of said stator and is adapted to be connected at the other end to a receiver or transmitter; and means operatively associated with said rotor for rotating the same relative to said stator, as the feed element moves along said arc-shaped aperture, in such manner that the beam of said feed element is always effectively directed at the center of said aperture through which energy may be emitted or received.

8. An antenna including a folded, parallel plate structure terminated at one end in a rst aperture through which energy may be emitted or received and terminated CII at another end in an arc-shaped second aperture along which a feed element may move, the radius of curvature of said arc being substantially shorter than the distance through the parellel plates from said second aperture to said rst aperture; a feed element for radiating or receiving energy in a directive beam including a stator formed with a pair of arc-shaped parallel plates aligned with the corresponding lparallel plates of said second aperture and mounted to e rotated along the are of said second aperture, and a rotor mounted in said stator formed with an internal waveguide therein having a horn-shaped opening at one end which communicates with the parallel plates of said stator, and another opening at its other end which is adapted to be connected to a receiver or transmitter; and mechanical means operatively associated with said rotor for rotating the same relative to said stator, as said feed element moves along said second aperture, in such manner that the beam of said feed element is always eectively directed at the center of said rst aperture.

References Cited in the tile of this patent UNITED STATES PATENTS 2,585,562 Lewis Feb. l2, 1952 

